Wigner-crystal and bi-stripe models for the magnetic and crystallographic superstructures of La0.333Ca0.667MnO3

The crystallographic (charge-ordered) and magnetic superstructures of La0.333Ca0.667MnO3 were studied by high-resolution synchrotron x-ray and neutron powder diffraction. In the antiferromagnetic structure, which was refined using a noncollinear model, the a lattice parameter is trifled and the c lattice parameter is doubled with respect to the average crystallographic unit cell (Pnma setting). The crystallographic structure below the charge-ordering temperature (T(CO)similar to 260 K) is characterized by ordering of the d(z2) orbitals of the Jahn-Teller-distorted Mn3+O6 octahedra in the orthorhombic ne plane, and the appearance of superlattice peaks in the x-ray patterns corresponding to a tripling of the a axis lattice parameter. The intensities of the superlattice peaks can be accounted for satisfactorily in terms of ordering of the Mn3+ cations in sites as far apart as possible in the ac plane ("Wigner-crystal" model) and transverse displacements of the Mn4+O6 octahedra in the c direction. These results are not consistent with a recently proposed model based on transmission electron microscopy (TEM) data in which the Mn3+O6 octahedra are arranged in a bi-stripe pattern ("bi-stripe" model). In particular, the large longitudinal stripe modulation revealed by TEM is not observed, suggesting that the TEM data may not be representative of the bulk sample. Within the framework of the Wigner-crystal model, the magnetic structure at 1.5 K and the charge-ordered superstructures at 160 and 1.5 K were refined from the neutron data. [S0163-1829(99)04421-5].